1. Field of the Invention
The present invention relates to an image blur prevention apparatus for preventing image blur which may occur due to camera shake or the like in a camera, an optical apparatus or the like.
2. Description of the Related Art
The prior art targeted by the present invention will be described below by using a camera as an example.
Cameras at the present time have become increasingly automated, and the possibility that persons unskilled in the handling of cameras will fail in photo-taking is very small. However, it has been difficult to automatically prevent a failure in photo-taking due to camera shake.
Therefore, in recent years, research and development of cameras for the purpose of preventing a failure in photo-taking due to camera shake of a photographer has been vigorously under way.
As a method of suppressing camera shake caused by a photographer, there is a method in which vibrations due to camera shake are detected, and a correction lens is displaced in correspondence with the detected value in order to correct the optical displacement of the image. To describe in more detail, shake vibrations are detected by vibration sensors, such as accelerometers or angular speedometers, angular velocity is detected electrically or mechanically on the basis of sensor signals, the correction optical system (which is a part of or the whole of the photographic optical system) is parallelly off-set in a direction at right angles to the optical axis on the basis of the detected information, or a correction lens is displaced in a direction different from the optical axis, for example, the optical axis is inclined.
As a method of driving a correction optical system, when the correction optical system is parallelly off-set in a direction at right angles to the optical axis, a connection member is secured to a fixation section of a photographic apparatus in such a manner as to be movable in a direction (e.g., up and down) at right angles to the optical axis, and a correction lens frame is secured to the connection member, to which correction lens frame a correction lens is secured so as to be movable in a different direction (e.g., from side to side) at right angles to the optical axis. Thus, the correction optical system can be moved in any direction at right angles to the optical axis with respect to the fixation section.
As an actuator for driving a correction lens, two electromagnetic coils corresponding to the above-described two directions at right angles to the optical axis are mounted in the correction lens frame, and a yoke and a permanent magnet in a pair are mounted in correspondence with the two coils in the fixation section. In this way, a moving coil type actuator is formed which is capable of controlling two direction components independently of each other.
Further, the position of the correction lens is detected, and the positional information is fed back, thereby realizing drive control of the correction lens with a high degree of accuracy.
In the above, shake correction employing a correction optical system has been described. After photographing is terminated, it is necessary to hold the correction optical system at a predetermined position. In a case where, in particular, the correction optical system moves in any direction because of the weight of the system itself as a result of the shut-off of the power supply, as in a moving coil type, a lock mechanism is especially required. When the fact that photographing is performed without correction of shake is taken into consideration, it is preferable that the correction optical system is held at the position where the optical axis of the correction optical system is made to coincide with the optical axis of the whole photographic optical system in terms of the position with the greatest optical performance.
Conventional examples of a lock mechanism formed to achieve such an object:
(1) A lock member is provided for holding the correction optical system at a neutral position, and the lock member is urged in a direction on which it is locked by a spring. When shake is to be corrected, the lock member is retracted from the lock position in opposition to the urging force of the spring by means of a solenoid so that the state in which the correction lens can be moved is maintained. When the correction of shake is terminated, by shutting off the supply of electric current to the solenoid, the lock member is driven to the lock position by the urging force of the spring, and the correction optical system is held at the neutral position again.
(2) By making the lock member for holding the correction optical system at the neutral position retract from the lock position by a motor, a shake correctable state is formed. By making the motor rotate reversely (or, further rotating), the lock member is driven to the lock position.
(3) A similar lock member is previously urged by a spring in a direction in which it is locked. When the shake is to be corrected, while the lock member is kept retracted from the lock position by a motor, the lock member is locked by a hook, and the state in which the correction optical system can be moved is maintained. When the correction of shake is terminated, the motor is slightly moved reversely, and the locking of the hook is released. In this way, the lock member is driven to the lock position by the spring. In this case, even if the power supply is discontinued suddenly while the correction of shake is being performed, the lock member can be locked by merely making the motor slightly rotate reversely by a back-up power supply.
(4) A bistable plunger is provided. When no correction of shake is to be performed, the correction lens is driven to the neutral position by a conventional actuator for correcting shake. In that state, a projection provided in the fixation section is inserted by the plunger into a hole provided in the correction lens frame, and the correction lens is locked.
However, the conventional examples have the following problems.
In the conventional example (1) described above, the solenoid requires a huge amount of electric power in order to drive the lock member by the solenoid to the state in which shake can be corrected. If less electric power is to be used, the shake correction range (correction angle) must be made narrower.
In the conventional examples (2) and (4) described above, if the power supply is discontinued suddenly while the correction of shake is being performed, the correction optical system cannot be locked. Not only can the subsequent photographing not be performed, but also the correction optical system which is not locked becomes unsteady because of the movement of the apparatus. There is even the risk that the apparatus might be damaged.
In the conventional example (3) described above, it is possible to lock the correction optical system even when the power supply is suddenly turned off. However, a capacitor serving as a back-up power supply is physically too large to be disposed in, in particular, a camera or a lens barrel, impeding the compactness of the apparatus.